This invention relates generally to heat pumps. More particularly the invention relates to heat pumps which are based on the principle of Stirling's air engine and intended to work in the temperature interval from -30.degree. to +100.degree. C.
Heat pumps are advantageously used where heat has to be transferred from one temperature level to another higher temperature level.
Heat pumps based on the vapor compression cycle are well known in the art. In these pumps energy losses occur as a result of irreversible processes which reduce their coefficient of performance. Commercial heat pumps may have coefficients of performance of between 2.3 and 2.5 when transferring heat from a temperature of 0.degree. C to a temperature between 32.degree. and 35.degree. C. The theoretical value for their coefficient of performance would be 9.
Apart from this poor coefficient of performance conventional heat pumps have the drawback that the temperature of the heat source has a lower limit of say -5.degree. C in air-to-air heat pumps. Another objection is that the working media are environmental pollutants and that in operation the pumps generate considerable noise.
In the art of the liquefaction of gases, particularly the liquefaction of air, a known type of refrigerating machine is that based on Stirling' s principle of operation. Such machines have not as yet been rendered suitable for use as heat pumps because irreversible losses due to friction between the pistons and the cylinder walls and to the resistance to gas flow through the regenerator are insupportably high.
The performance of a heat pump can be calculated from the following formula ##EQU1## where Q is the heat rejected in watts, n is the recirculated volume of gas in mols per second, R is the gas constant (R = 8.31 wattsecs/g.mol), T is the heating temperature level in .degree.K and V.sub.2 /V.sub.1 is the compression ratio by volume which may here be assumed to be 2.
It will therefore be understood that the thermal output depends upon n. A large volume of gas n means a large cylinder volume or a smaller volume at higher gas pressure. Moreover, the thermal output can also be improved by raising the frequency of piston reciprocation.
Conventional gas liquefaction plants operate with a small volume of gas at higher pressure and high frequencies of piston reciprocation. This is not practicable for a Stirling type heat pump because the heat exchangers required for a domestic air conditioning system would call foor a very large working volume. Naturally large heat pumps would have large pistons and the frictional losses would be correspondingly high.